U.S. patent number 4,244,934 [Application Number 06/033,658] was granted by the patent office on 1981-01-13 for process for producing flexible graphite product.
Invention is credited to Jiro Ishiguro, Teruhisa Kondo, Nobuatsu Watanabe.
United States Patent |
4,244,934 |
Kondo , et al. |
January 13, 1981 |
Process for producing flexible graphite product
Abstract
Wet graphite particles obtained by subjecting graphite particles
to oxidizing treatment with nitric acid of comparatively low
concentration and a permanganic acid salt and impregnating the
treated graphite particles with a specific antioxidant comprising a
metal salt of a boric acid ester of a saccharide, are heated to
form expanded graphite masses, which are then compressed together
to obtain a flexible graphite product. The thus obtained flexible
graphite product is not only excellent in heat resistance and
chemical inertness but also free of contamination with sulfur and
chlorine, and is very useful in various applications.
Inventors: |
Kondo; Teruhisa (Toyonaka-shi,
Osaka-fu, JP), Ishiguro; Jiro (Ooeda, Ooaza,
Kasukabe-shi, Saitama-ken, JP), Watanabe; Nobuatsu
(Nagaokakyo-shi, Kyoto-fu, JP) |
Family
ID: |
15457492 |
Appl.
No.: |
06/033,658 |
Filed: |
April 26, 1979 |
Foreign Application Priority Data
|
|
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|
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Dec 2, 1978 [JP] |
|
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53-148648 |
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Current U.S.
Class: |
423/448;
264/109 |
Current CPC
Class: |
C04B
35/536 (20130101) |
Current International
Class: |
C04B
35/536 (20060101); C01B 031/04 () |
Field of
Search: |
;264/109,343
;423/448 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Czaja; Donald E.
Assistant Examiner: Hall; J. R.
Attorney, Agent or Firm: Sprung, Felfe, Horn, Lynch &
Kramer
Claims
What is claimed is:
1. A process for producing a flexible graphite product which
comprises the steps of:
(1) subjecting graphite particles to oxidizing treatment with
nitric acid having a concentration of about 60 to about 95% by
weight and a permanganic acid salt;
(2) washing the treated graphite particles with water;
(3) contacting the washed graphite particles with an antioxidant
comprising at least one metal salt of a boric acid ester of a
member selected from the group consisting of a reduction product of
a monosaccharide, a dimer of a monosaccharide and combinations
thereof for a sufficient time for said antioxidant to permeate said
washed graphite particles, said metal being a member selected from
metals of the group II of the periodic table;
(4) heating the resulting graphite particles to obtain expanded
graphite masses; and
(5) compressing the expanded graphite masses to form a flexible
graphite product.
2. A process according to claim 1, wherein said reduction product
of a monosaccharide is a member selected from the group consisting
of sorbitol, mannitol and combinations thereof and said dimer of a
monosaccharide is a member selected from the group consisting of
sucrose, maltose, lactose and combinations thereof, and said metal
is a member selected from the group consisting of magnesium,
calcium, zinc and barium.
3. A process according to claim 1, wherein said antioxidant is an
aqueous solution of said at least one metal salt.
4. A process for producing a flexible graphite product which
comprises the steps of:
(1) dispersing graphite particles in nitric acid having a
concentration of about 60 to about 95% by weight to form a slurry
of the graphite particles;
(2) adding a permanganic acid salt to said slurry to effect
oxidation of said graphite;
(3) washing the oxidized graphite particles with water;
(4) contacting the washed graphite particles with an antioxidant
comprising at least one metal salt of a boric acid ester of a
member selected from the group consisting of a reduction product of
a monosaccharide, a dimer of a monosaccharide and combinations
thereof for a sufficient time for said antioxidant to permeate said
washed graphite particles, said metal being a member slected from
metals of the group II of the periodic table;
(5) heating the resulting graphite particles to obtain expanded
graphite masses; and
(6) compressing the expanded graphite masses to form a flexible
graphite product.
5. A process according to claim 4, wherein the permanganic acid
salt is employed in an amount of about 4 to about 20% by weight
based on the graphite particles.
6. A flexible graphite product produced by a process of claim
1.
7. A flexible graphite product produced by a process of claim
2.
8. A flexible graphite product produced by a process of claim
3.
9. A flexible graphite product produced by a process of claim
4.
10. A flexible graphite product produced by a process of claim 5.
Description
This invention relates to a process for producing a flexible
graphite product. More particularly, the present invention is
concerned with a process according to which there can be produced a
flexible graphite product that is not only excellent in heat
resistance and chemical inertness but also free of contamination
with sulfur and chlorine.
Heretofore, various processes have been proposed for producing
flexible graphite products. The known processes generally consist
in expanding graphite particles to large extent with respect to a
bulk density ratio and compressing the resulting expanded graphite
particles into a graphite sheet material, followed by fabrication
to form a graphite product having a desired shape. For example,
U.S. Pat. No. 3,404,061 discloses a process in which particles of
natural graphite, kish graphite or pyrolytic graphite are immersed
in an oxidizing bath comprising concentrated sulfuric acid and
concentrated nitric acid under suitable temperature and time
conditions and the resulting wetted graphite particles are, upon
washing with water, heated to about 1,000.degree. C. to obtain
vermiform expanded graphite particles of which the dimension in the
c direction (the direction perpendicular to the layers of carbon
atoms) is expanded up to 80 times or more, preperably 100 to 300
times the original dimension, followed by compressing of the
expanded graphite masses in the absence of a binder or agglutinant
to form a graphite sheet material as a flexible graphite
product.
Furthermore, with respect to a process for producing a flexible
graphite product having an improved oxidation-resistance, there is
known a process as disclosed in Japanese laid-open patent
application specification No. 35205/1977. According to the process
as mentioned above, graphite particles are immersed in an oxidizing
bath comprising concentrated sulfuric acid and concentrated nitric
acid at room temperature for a suitable period of time and
subsequently the graphite particles are washed with water until the
pH value of the graphite particles becomes 4 to 7, thereby to
obtain wet graphite particles. The thus obtained wet graphite
particles are subjected to antioxidizing treatment, that is, they
are dipped in a 0.05 to 1.0 mole concentration aqueous solution of
phosphoric acid or a suitable phosphate for 0.5 to 10 hours. The
thus treated wet graphite particles are then expanded by heat
treatment at 600.degree. to 800.degree. C. in air under atmospheric
pressure. In this instance, expansion is effected so that the bulk
density of the expanded graphite particles is 1/20 to 1/70 of that
of the original graphite particles. Subsequently, the expanded
graphite masses are compressed or molded into a flexible graphite
product.
The flexible graphite products prepared by the conventional
processes are still insufficient in oxidiation-resistance.
Furthermore, as is apparent from the above, in the known processes,
for obtaining wet graphite particles by oxidizing treatment, there
is generally employed an oxidizing medium containing concentrated
sulfuric acid having a concentration as high as 95 to 98%. Examples
of the conventionally employed oxidizing media include mixtures of
concentrated sulfuric acid with concentrated nitric acid,
perchloric acid, chromic acid, potassium permanganate, iodic acid
or periodic acid. Among them, a mixture of concentrated sulfuric
acid with concentrated nitric acid or perchloric acid is often
used. In any the conventional oxidizing treatments, concentrated
sulfuric acid is used and, hence, there are caused various
problems, for example, danger in operation, difficulty in disposal
of spent oxidizing medium, high cost, etc.
In the conventional process for producing a flexible graphite
product which comprises treating graphite particles with an
oxidizing agent comprising, for example, concentrated sulfuric acid
and perchloric acid to obtain wet graphite particles,
heat-expanding the wet graphite particles to produce vermiform
graphite masses and compressing the vermiform graphite masses to
form a flexible graphite product, there is caused such a problem
that in the flexible graphite product, a sulfur compound and a
chlorine compound inevitably remain unremoved. Such remaining of
the sulfur and chlorine compounds is caused by the use of special
heating conditions for the expansion of the oxidized, wet graphite
particles. Illustratively stated, the heat treatment of the wet
graphite particles for expansion thereof is conducted at high
temperatures but for a short time and, therefore, after the heat
treatment, considerable quantities of a sulfur compound (--SO.sub.4
H) and a chlorine compound remain in and are fixed to the expanded
graphite masses. Even if in order to eliminate the remaining
compounds of sulfur and chlorine the graphite masses are subjected
to a long time heat treatment at temperatures in the range where
the graphite masses are not subject to oxidization with oxygen in
air, it is difficult to completely eliminate the remaining
compounds of sulfur and chlorine. If a flexible graphite product
contaminated with the compounds of sulfur and chlorine is used as a
sealing material or the like, the sulfur compound and chlorine
compound tend to destroy or dissolve even the passive-state,
corrosion-resistant surface layer of a metal material, e.g.,
stainless steel which is in contact with the flexible graphite
product, thus causing corrosion of the metal material layer to
occur and promoting progress of the corrosion, which leads to
serious problems in the practical use of the flexible graphite
material. Furthermore, flexible graphite products contaminated with
the compounds of sulfur and chlorine have another drawback that
when they are used in the field of electrochemistry, for example,
as an electrode, the sulfur and chlorine values as the contaminants
have an adverse effect on the intended function of the graphite
product.
With a view to obviating the above-mentioned drawbacks inevitaly
accompanying the conventional processes of producing flexible
graphite products and to providing flexible graphite products
having excellent properties, the inventors of the present invention
have made extensive and intensive investigations. As a result, it
has been found that by a process in which graphite particles are
subjected to oxidizing treatment with nitric acid of comparatively
low concentration and a permanganic acid salt, the resultant is
treated with an antioxidant comprising at least one metal salt of a
boric acid ester of a member selected from the group consisting of
a reduction product of a monosaccharide, a dimer of a
monosaccharide and combinations thereof, said metal being a member
selected from metals of the group II of the periodic table, and the
resulting wet graphite particles are expanded by heat treatment to
obtain expanded graphite masses, followed by compression, there is
obtained a flexible graphite product which is not only excellent in
oxidation-resistance, tensile strength, impermeability to gases and
handling characteristics but also is completely free of
contamination with compounds of sulfur and chlorine. Based upon
such a novel finding, the present invention has been made.
Accordingly, it is an object of the present invention to provide a
process according to which there can be produced a flexible
graphite product that is not only excellent in heat resistance and
chemical inertness but also free of contamination with compounds of
sulfur and chlorine.
It is another object of the present invention to provide a process
as mentioned above, which can be conducted with ease and
convenience due to the combined use of nitric acid of comparatively
low concentration and a permanganic acid salt in the oxidizing
treatment.
It is a further object of the present invention to provide a
process of the character described, in which the nitric acid used
in the oxidizing treatment can be effectively recovered and can be
advantageously recycled for repeated use.
The foregoing and other objects, features and advantages of the
present invention will be apparent to those skilled in the art from
the following detailed description and appended claims.
According to the present invention, there is provided a process for
producing a flexible graphite product which comprises the steps of
(1) subjecting graphite particles to oxidizing treatment with
nitric acid of comparatively low concentration and a permanganic
acid salt, (2) washing the treated graphite particles with water,
(3) contacting the washed graphite particles with an antioxidant
comprising at least one metal salt of a boric acid ester of a
member selected from a reduction product of a monosaccharide, a
dimer of a monosaccharide and combinations thereof for a sufficient
time for said antioxidant to permeate said washed graphite
particles, said metal being a member selected from metals of the
group II of the periodic table, (4) heating the resulting graphite
particles to obtain expanded graphite masses, and (5) molding,
under pressure, the expanded graphite masses to form a flexible
graphite product.
Examples of the graphite particles employable in the process of the
present invention include natural graphite, Kish graphite and
synthetic graphites such as pyrolytic graphites. Of them, natural
flake graphites are preferrably employed, and those having a sieve
size of about 5 to 100 mesh (Tyler) and a high purity are
especially preferred from a viewpoint of good quality of the final
graphite product as well as ease and convenience in respective
operations of the oxidizing treatment, washing with water,
impregnation with the metal salt of the boric acid ester, heat
treatment for expansion and compression molding.
A concentration of the nitric acid of comparatively low
concentration to be employed in the process of the present
invention may be about 60 to about 95%, preferably about 75 to
about 80% by weight. An amount of the nitric acid of comparatively
low concentration is not critical, and the nitric acid may be
employed in an amount sufficient to well disperse therein the
graphite particles so that a slurry of the graphite particles is
formed. In general, the nitric acid may preferably be employed in
an amount of about 300 to 500% by weight based on the amount of
graphite particles.
Preferred examples of the permanganic acid salt to be employed in
the process of the present invention include potassium
permanganate, sodium permanganate and ammonium permanganate. An
amount of the permanganic acid salt may be about 4 to about 20% by
weight, preferably about 7 to about 10% by weight based on the
amount of graphite particles.
Temperature to be employed in the oxidizing treatment of the step
(1) as mentioned above may be room temperature to temperatures
below the boiling point of the nitric acid of a predetermined
concentration. Period of time of the oxidizing treatment may be
about 1 to 5 hours, preferably about 2 to 4 hours. After the
oxidizing treatment, the treated graphite particles are washed in
the step (2) as mentioned above. Washing with water is rapidly
conducted until the pH value of the washings becomes not less than
2, preferably not less than 4.
In the step (3) of the process of the present invention, the
water-washed graphite particles are subjected to antioxidizing
treatment with an antioxidant comprising at least one metal salt of
a boric acid ester of a member selected from the group consisting
of a reduction product of a monosaccharide, a dimer of a
monosaccharide and combinations thereof, said metal being a member
selected from metals of the group II of the periodic table.
As examples of the reduction product of a monosaccharide, there can
be mentioned sorbitol and mannitol which are reduction products of
dextrose and D-fructose, respectively. The dimers of
monosaccharides are those which are generally called
"disaccharides", and include, for example, sucrose, maltose and
lactose. Among metals of the group II of the periodic table, there
may advantageously be employed magnesium, calcium, zinc, barium and
the like. Such polyhydroxy compounds as the reduction products of
monosaccharides and the dimers of monosaccharides (hereinafter,
both often referred to simply as "saccharide") may be employed
alone or in combination. The metals of the group II of the periodic
table may also be employed alone or in combination.
The above-defined metal salts of boric acid esters of saccharides
to be employed in the present invention are novel compounds. These
novel compounds can be prepared by a process in which a saccharide
that is a polyhydroxy compound as defined above is reacted with
boric acid and the resulting boric acid ester of saccharide is then
neutralized with a compound of a metal of the group II of the
periodic table. The thus obtained metal salts of boric acid esters
of saccharides are soluble or compraratively soluble in water as
opposed to the salts of boric acid with metals of the group II of
the periodic table which are insoluble or sparingly soluble in
water. Therefore, the metal salts of boric acid esters of
saccharides can be easily formulated into an aqueous solution
thereof, and hence, can advantageously be used for the
antioxidizing treatment in the step (3) of the process according to
the present invention. As representative examples of the compounds
of metals of the group II of the periodic table, there can be
mentioned oxides, hydroxides, carbonates and basic carbonates of
magnesium, calcium, zinc and barium. As preferred examples of the
basic carbonate, there can be mentioned
4MgCO.sub.3.Mg(OH).sub.2.5H.sub.2 O,
3MgCO.sub.3.Mg(OH).sub.2.5H.sub.2 O and mixtures thereof.
The metal salt of a boric acid ester of a saccharide may be
prepared as follows. A saccharide selected from the group
consisting of a reduction product of a monosaccharide, a dimer of a
monosaccharide and mixtures thereof is mixed with water. An amount
of water to be used may be about 30 to 100% by weight, preferably
about 50 to 60% by weight, based on the saccharide employed. To the
resulting mixture or solution is added boric acid to effect
esterification reaction. An amount of boric acid to be added may be
about 0.5 to 2 moles, preferably about 0.5 to 1.5 mole, more
preferably about 1.0 mole per mole of the saccharide employed. The
esterification reaction may be conducted under the ordinary
reaction conditions for esterification with dehydration. The
reaction temperature may be about 105.degree. to 130.degree. C.,
preferably 110.degree. to 120.degree. C. The reaction period may
vary depending on the reaction temperature, but may generally be
within the range of about 2 to 5 hours. As the reaction product,
there is obtained a boric acid ester of the saccharide in the form
of a transparent, viscous liquid. The thus obtained boric acid
ester is diluted with water so that there is obtained an aqueous
solution of the ester having a solid concentration of 30 to 80% by
weight, preferably 40 to 60% by weight. To the resulting aqueous
solution of the boric acid ester is added a compound of a metal of
the group II of the periodic table in an amount of about 1 to 3
moles (in terms of amount of metal oxide) per mole (in terms of
amount of boric acid anhydride) of the boric acid ester. In this
connection, it is noted that the content of boron in the boric acid
ester can be easily, accurately determined because the boric acid
ester formation is quantitative. The reaction temperature for the
formation of a metal salt of the boric acid ester by the reaction
between the boric acid ester and the metal compound is not
critical, and the reaction may proceed sufficiently at about room
temperature. If desired, however, the reaction may be effected at
elevated temperatures, so that the rate of reaction can be
advantageously increased. In general, the reaction for the
formation of a metal salt of the boric acid ester may be conducted
at temperatures ranging from about room temperature to about
100.degree. C. By conducting the reaction, while stirring, for
about 20 to 30 minutes, there is obtained a desired metal salt of a
boric acid ester of a saccharide in the form of a transparent
aqueous solution. The solid concentration of the thus obtained
aqueous solution of a metal salt of a boric acid ester of a
saccharide is varied depending on the atomic weight of the metal
incorporated as the salt of the boric acid ester, but may in
general be within the range of about 50 to about 60% by weight. The
above-mentioned solid concentration can be easily determined by the
so-called xylene method.
An explanation on the mechanism of the above-mentioned formation of
a metal salt of a boric acid ester of a saccharide will now be
given, referring, for example, to the case where sucrose is
employed as the saccharide. As well known, sucrose is a polyhydroxy
compound represented by the following formula ##STR1## The hydroxyl
group in sucrose is reacted with boric acid of the formula ##STR2##
to effect esterification with dehydration, thereby forming an ester
linkage represented by the formula ##STR3## The formation of the
ester linkage is believed to occur at one or some of the three
--CH.sub.2 OH groups in sucrose. After formation of the ester
linkage, the hydrogen moieties(acidic) in the above-mentioned boric
acid ester structure are neutralized with a compound of a metal of
the group II of the periodic table, which compound is selected
from, for example, oxides, hydroxides, carbonates and basic
carbonates of magnesium, calcium, zinc and barium. Thus, there is
obtained a metal salt of a boric acid ester of sucrose which is a
novel compound and useful as one of antioxidants to be used in the
step (3) of the process according to the present invention.
A metal salt of a boric acid ester of a saccharide prepared
according to the process as mentioned above is completely
water-soluble, where the molar ratio of the boric acid values to
the saccharide values in the metal salt is 1(one) or less. Where
the above-mentioned molar ratio is more than 1(one), the metal salt
is not so completely water-soluble but partially dispersed in water
(i.e., comparatively water-soluble). Even the metal salt which is
comparatively water-soluble as mentioned above, however, can be
used, without any trouble, in the form of an aqueous solution in
which the metal salt is partially dispersed (hereinafter, such as
aqueous solution is often referred to simply as "aqueous
solution"), as the antioxidant in the step (3) of the process of
the present invention and the intended purpose can be attained by
the use of such a comparatively water-soluble metal salt.
As mentioned above, a metal salt of a boric acid ester of a
saccharide prepared according to the procedures as described above
is water soluble or comparatively water-soluble and, therefore, can
be easily used in the form of an aqueous solution for contacting
the graphite particles therewith.
The contacting of the graphite particles with the antioxidant in
the step (3) of the process of the present invention may be
effected by impregnation. The impregnation may be conducted using a
3 to 30% by weight aqueous solution of a metal salt of a boric acid
ester of a saccharide. The method for impregnation is not limited,
and any of the ordinarily employed impregnation methods such as
dipping, brushing, spraying, etc. can be utilized. From a view
point of efficiency of working, a dipping method may preferably be
used. In the dipping method, pressure is not critical, and dipping
may be conducted under atmospheric pressure, super atmospheric
pressure or reduced pressure. Dipping time is also not critical and
dipping may be made for a sufficient time for the antioxidant to
permeate the graphite particles. Dipping time may generally be
within the range of about 30 minutes to several-ten hours.
The antioxidant-treated graphite particles are then heated to
obtain expanded graphite masses. Heating temperature may suitably
be within the range of about 400.degree. to 1,300.degree. C.,
preferably about 600.degree. to about 1,000.degree. C. Heating time
may be varied depending on the heating temperature as well as the
size of the graphite particles, but may generally be in the range
of several seconds to ten and several hours. The thus obtained
expanded graphite masses are molded under pressure to form a
desired flexible graphite product. The heat treatment for expansion
of the graphite in the step (4) may be done until the graphite
particles are expanded to give expanded graphite masses having a
bulk density of 0.005 to 0.015. The pressure molding in the step
(5) may be conducted under a pressure of about 80 to about 400
Kg/cm.sup.2, preferably about 100 to about 200 Kg/cm.sup.2.
In the heat treatment in the step (4), according to the present
invention, the organic moiety values in the metal salt of the boric
acid ester of a saccharide are carbonized and the residual carbon
thus formed is believed to serve, in the step (5) of molding under
pressure, as a binder for the graphite masses, enabling an
excellent flexible graphite product to be obtained without any use
of additional binder material. Furthermore, when the graphite
particles impregnated with a metal salt of a boric acid ester of a
saccharide is heated, the metal salt is caused to form a
two-component system glassy or ceramic compound, namely a B.sub.2
O.sub.3 -metal oxide system compound. Such a two-component system
glassy or ceramic compound is chemically and firmly adsorbed onto
the active sites in the interior of the carbonaceous material to
form a caking structure. It is believed that the thus formed caking
structure serves to effectively impart a high oxidation-resistance
to the flexible graphite product.
An illustrative explanation on the procedures embodying the process
of the present invention will now be given as follows.
Graphite particles are dispersed in nitric acid of a concentration
of about 60 to about 95% by weight to make a slurry of the graphite
particles. To the slurry is portion-wise added, while stirring, a
permanganic acid salt such as potassium permanganate in an amount
of about 4 to 20% by weight based on the graphite particles,
thereby to effect oxidizing treatment. In this connection, it is
noted that there may alternatively be employed such an operation
that a mixture of nitric acid and a permanganic acid salt is
prepared, and graphite particles are then immersed in the mixture
to effect oxidizing treatment of the graphite particles. In such an
operation, however, when a permanganic acid salt is added to nitric
acid, the vapor of permanganic acid is generated. On the other
hand, even if a mixture of nitric acid and a permanganic acid salt
can be prepared by very carefully, very slowly adding the latter to
the former, the mixture is instable and, hence, when graphite
particles are added into the mixture the oxidizing reaction is
caused to violently occur so that the oxidation of the graphite
particles tends to be non-uniform, leading to poor quality of the
final product. Therefore, the use of a mixture of nitric acid and a
permanganic acid salt from the beginning is not recommended from a
practical point of view, but the two-stage operation, namely, first
preparing of a slurry of the graphite particles in nitric acid and
secondly portion-wise adding of a permanganic acid salt to the
slurry, is preferably conducted.
The treated graphite particles are washed with water until the pH
value of the washings becomes not less than 2, preferably not less
than 4. The thus obtained graphite perticles are dipped in a 2 to
30 weight % aqueous solution of a boric acid ester of a saccharide
such as magnesium salt of the boric acid ester of sucrose for 30
minutes to several-ten hours. After the graphite particles are
dipped in an aqueous solution of a metal salt of a boric acid ester
of a saccharide, the content of the aqueous solution in the
carbonaceous material may preferably be reduced to about 30 to
about 50% by weight by a suitable method such as suction
filtration, centrifugation or the like. Subsequently, the graphite
particles are dried at temperatures of not higher than 100.degree.
C., and then subjected to heat-treatment at about 400.degree. to
about 1,300.degree. C., preferably about 600.degree. to
1,000.degree. C. to obtain expanded graphite masses having a bulk
density of about 0.005 to 0.015. The expanded graphite masses are
subjected to molding under pressure, for example, compression
molding and/or roll molding to obtain a desired shape of flexible
graphite product having a specific gravity of about 0.8 to 1.8. In
this connection, it is noted that the expanded graphite masses
which have been treated with the antioxidizing agent comprising a
metal salt of the boric acid ester can be advantageously molded,
without incorporation thereinto of any additional binder or
agglutinant, into a flexible graphite product having a high
mechanical strength.
In the oxidizing treatment, the co-use of a permanganic acid salt
and nitric acid is advantageous because even though the
concentration of the nitric acid is comparatively low there can be
obtained wet graphite particles which are capable of being
sufficiently expanded by heating. In order to obtain sufficiently
expandable graphite particles by the treatment of graphite
particles with only nitric acid, it is necessry to employ high
concentration nitric acid, i.e. fuming nitric acid. In that case,
there is unavoidably generated large quantities of nitrogen oxides,
the treatment of which is not only troublesome but also costly. By
the co-use of comparatively low concentration nitric acid and a
permanganic acid salt, according to the present invention, the
above-mentioned disadvantages inevitably accompanying the
conventional process in which high concentration nitric acid is
employed as the oxidizing agent, can be effectively obviated.
As described, according to the process of the present invention,
after graphite particles are treated with comparatively low
concentration nitric acid and a permanganic acid salt, the treated
graphite particles are subjected to antioxidizing treatment with
antioxidant comprising a specific metal salt of a boric acid ester
of a saccharide and heated for expansion to have a bulk density of
0.005 to 0.015, followed by compression-molding and/or roll-molding
of the resulting expanded graphite masses to form a flexible
graphite product. The thus formed graphite product is not only
excellent in heat resistance and chemical inertness but also free
of contamination with compounds of sulfur and chlorine. The freedom
of the contamination leads to such an advantage that a flexible
graphite sheet material prepared according to the process of the
present invention can be advantageously used as a sealant without
fear of causing corrosion of a metal material which is in contact
with the sealant, and as a material for the electrochemical
applications. Furthermore, a specific metal salt of a boric acid
ester of a saccharide incorporated according to the process of the
present invention serves to not only increase mechanical bonding
strength in the heat-treated graphite particles due to the
carbonaceous material formed by thermal decomposition of the metal
salt of the boric acid ester, but also impart a high
oxidation-resistance to the graphite due to the ceramic compound of
a boron-metal system formed by the heat treatment so that an
oxidation loss of the graphite is minimized not only at the step of
the heat treatment in the process of the present invention but also
at the time when the final flexible graphite product is exposed to
temperatures as high as 500.degree. C. or more.
In addition, it should be noted that the comparatively low
concentration nitric acid used according to the process of the
present invention can, after once used, be advantageously recycled,
with replenishment of a suitable amount of a permanganic acid salt,
so that the nitric acid can be reutilized for at least 10-time
operations, occasionally for operations as many as up to about 20
times.
The present invention is further illustrated in more detail by the
following examples, which should not be construed to be limiting
the scope of the present invention.
The metal salts of boric acid esters of saccharides employed in
Examples were prepared according to the methods shown in
Referential Examples, respectively.
In Examples and Comparative Examples, measurement of a tensile
strength was done substantially according to the method prescribed
in Japanese Industrial Standard (JIS) K-6301-1975, item 4, and
measurement of a specific gravity was done, using a 15 mm.times.60
mm.times.0.4 mm sample piece, according to the voltage dropping
method prescribed in Japan Carbon Association Standard
(JCAS-15-1971), item 6-1-6 (b).
REFERENTIAL EXAMPLE 1
To 260 g(1 mole in terms of sorbitol) of SORBIT D-70 (trade name of
a 70 weight % aqueous solution of sorbitol manufactured and sold by
Towa Kasei Kogyo Kabushiki Kaisha, Japan) were added 61.8 g(1 mole)
of boric acid. The resulting mixture was heated, while stirring, at
110.degree. to 120.degree. C. for 3 hours to effect esterification
with dehydration. The thus obtained boric acid ester of sorbitol
was diluted with water so as to form an aqueous solution of the
boric acid ester which solution had a solid content of 50% by
weight. To the aqueous solution were portion-wise added, with
agitation, 20.2 g(0.5 mole) of magnesium oxide to effect
neutralization reaction. There was obtained the desired magnesium
salt of boric acid ester of sorbitol. The thus obtained salt of the
boric acid ester was a transparent, viscous liquid and easily
soluble in water. The pH value of a 1 weight % aqueous solution of
the salt was 7.2.
REFERENTIAL EXAMPLE 2
A mixture of 130 g(0.5 mole in terms of sorbitol) of SORBIT D-70
and 171 g(0.5 mole) of sucrose was heated, with agitation, at
100.degree. C. to completely dissolve the sucrose therein. To the
thus obtained uniform mixture were added 61.8 g(1 mole) of boric
acid, and heating is then conducted at 115.degree. C. for 3 hours
to effect esterification with dehydration. The resulting ester was
a light brown, viscous and water-soluble substance. The ester was
diluted with water so as to have a solid content of 50% by weight,
and then, 20.2 g(0.5 mole) of magnesium oxide were portion-wise
added, with agitation, to effect neutralization reaction. There was
obtained the desired magnesium salt of the boric acid ester which
as also a light brown, viscous and water-soluble substance. The pH
value of a 1 weight % aqueous solution of the salt was 8.4.
REFERENTIAL EXAMPLE 3
To 342 g(1 mole) of sucrose were added 170 ml of water, followed by
heating to dissolve the sucrose in the water. To the resulting
solution were added 61.8 g(1 mole) of boric acid and then, heating
was conducted, with agitation, at 120.degree. C. for 3 hours to
effect esterification with dehydration. The resulting boric acid
ester of sucrose was a brown, viscous and water-soluble
substance.
The boric acid ester was diluted with water so as to have a solid
content of 50% by weight, and was then neutralized with 47 g(0.5
mole in terms of MgO) of basic magnesium carbonate to obtain the
desired magnesium salt of boric acid ester of sucrose which was
brown and water-soluble. The pH value of a 1 weight % aqueous
solution of the salt was 6.0.
REFERENTIAL EXAMPLE 4
The aqueous solution of the boric acid ester prepared in
substantially the same manner as described in Referential Example 1
was subjected to neutralization with 157.7 g(0.5 mole) of barium
hydroxide (octahydrate) to obtain a water-soluble, transparent,
viscous product which was the desired barium salt of the boric acid
ester. The pH value of a 1 weight % aqeuous solution of the barium
salt was 7.9. The diluted aqueous solution of the barium salt was
somewhat semi-turbid.
REFERENTIAL EXAMPLE 5
The aqueous solution of the boric acid ester prepared in
substantially the same manner as described in Referential Example 1
was subjected to neutralization with 37 g(0.5 mole) of calcium
hydroxide to obtain an extremely water-soluble, transparent,
viscous product which was the desired calcium salt of the boric
acid ester. The pH value of a 1 weight % aqueous solution of the
calcium salt was 7.9.
REFERENTIAL EXAMPLE 6
Substantially the same procedures of preparing a boric acid ester
as described in Referential Example 1 were repeated with the
exception that 182 g of mannitol were used in place of 260 g of
SORBIT D-70. There was obtained a boric acid ester of mannitol. The
thus obtained boric acid ester of mannitol was diluted with water
so as to have a solid content of 50% by weight. The resulting
aqueous solution of the boric acid ester was subjected to
neutralization with 157.7 g(0.5 mole) of barium
hydroxide(octahydrate). The thus obtained barium salt of the boric
acid ester was water-soluble. The pH value of a 1 weight % aqueous
solution of the barium salt was 9.0.
REFERENTIAL EXAMPLE 7
An aqueous solution (solid content: 50% by weight) of a boric acid
ester of mannitol prepared in substantially the same manner as
described in Referential Example 6 was subjected to neutralization
with 20.2 g(0.5 mole) of magnesium oxide to obtain the desired
magnesium salt of the boric acid ester of mannitol which was a
water-soluble, light orange-colored, viscous liquid. The pH value
of a 1% by weight aqueous solution of the magnesium salt was
8.8.
EXAMPLE 1
100 g of Madagascar-produced natural flake graphite particles
having a bulk density of 0.81 and a sieve size of 50 to 80 mesh
(Tyler) were dispersed in 400 g of 75% nitric acid, and then, 7 g
of potassium permanganate were portion-wise added, while stirring,
at 30.degree. C. As the potassium permanganate was added as
mentioned above, the temperature of the system rose to about
40.degree. C. After the addition of magnesium permanganate, the
system was heated, and maintained at 60.degree. C. for 2 hours to
accomplish oxidation reaction. The liquid in the system was removed
by centrifugation, and subsequently the resulting graphite
particles were washed with water so that the pH value thereof
became 6.0. The thus washed graphite particles were dehydrated by
centrifugation so that the water content thereof became about 30%,
to obtain wet graphite particles.
100 g of the wet graphite particles were dipped in 500 ml of an
aqueous solution (concentration: 3% by weight) of mangesium salt of
the boric acid ester of sucrose for 1 hour. The resulting soggy
graphite particles were subjected to suction filtration so that the
content of the aqueous solution was adjusted to 30% by weight.
The graphite particles treated as mentioned above were dried at
temperatures below 100.degree. C. and then heated in an electric
furnace at 1,000.degree. C. for 1 minute to obtain expanded,
vermiform graphite masses having a bulk density of 0.012.
5 g of the vermiform graphite masses were charged into a 100
mm.times.150 mm metal mould and then compression-molded under a
pressure of 100 Kg/cm.sup.2 by means of a pressing machine. The
compressed graphite masses were heated at 800.degree. C. and
further compressed under a pressure of 100 Kg/cm.sup.2, followed by
pressing by means of a constant speed roll so that the surface of
the product was flattened and smoothened. There was obtained a
flexible graphite sheet material having a thickness of 0.3 mm. A
sample piece (30 mm.times.60 mm) of the flexible graphite sheet
material was maintained at 600.degree. C. in an electric furnace
for 4 hours. From the weight difference between before and after
the heat treatment, there was calculated an oxidation loss. Results
are shown in Table 1, together with data of tensile strength and
specific resistance.
COMPARATIVE EXAMPLE 1
Instead of the aqueous solution of the magnesium salt of the boric
acid ester of sucrose, there was employed a 3 weight % aqueous
solution of phosphoric acid. Except for the above, substantially
the same procedures as described in Example 1 were repeated to
obtain a flexible graphite sheet. An oxidation loss of the sheet
was examined in the same manner as in Example 1. Results are also
shown in Table 1.
COMPARATIVE EXAMPLE 2
Substantially the same procedures as described in Example 1 were
repeated except that the antioxidizing treatment with the aqueous
solution of the magnesium salt of the boric acid ester of sucrose
was omitted and the wet graphite particles as prepared through the
oxidizing treatment in Example 1 were subjected directly to
preparation of graphite product and test in substantially the same
manner as described in Example 1. Results are also shown in Table
1.
EXAMPLE 2
100 g of China-produced natural flake graphite particles having a
bulk density of 0.67 and a sieve size of 50 to 100 mesh (Tyler)
were dispersed in 400 g of 80% nitric acid, and then, 10 g of
potassium permanganate were portion-wise added, while stirring, at
30.degree. C. As the potassium permanganate was added as mentioned
above, the temperature of the system rose by about 10.degree. C.
After the addition of magnesium permanganate, the system was
heated, and maintained at 60.degree. C. for 3 hours to accomplish
oxidation reaction. The liquid in the system was removed by
centrifugation, and subsequently the resulting graphite particles
were washed with water so that the pH value thereof became 5.0. The
thus washed graphite particles were dehydrated by centrifugation so
that the water content thereof became about 30%, to obtain wet
graphite particles.
100 g of the wet graphite particles were dipped in 500 ml of an
aqueous solution (concentration: 3% by weight) of magnesium salt of
the boric acid ester of sucrose for 1 hour. The resulting soggy
graphite particles were subjected to suction filtration so that the
content of the aqueous solution was adjusted to 30% by weight.
The graphite particles treated as mentioned above were dried at
temperatures below 100.degree. C. and then heated in an electric
furnace at 1,000.degree. C. for 1 minute to obtain expanded,
vermiform graphite masses having a bulk density of 0.010.
5 g of the vermiform graphite masses were charged into a 100
mm.times.150 mm metal mould and then compression-molded under a
pressure of 100 Kg/cm.sup.2 by means of a pressing machine. The
compressed graphite masses were heated at 800.degree. C. and
further compressed under a pressure of 100 Kg/cm.sup.2, followed by
pressing by means of a constant speed roll so that the surface of
the product was flattened and smoothened. There was obtained a
flexible graphite sheet material having a thickness of 0.3 mm. A
sample piece (30 mm.times.60 mm) of the flexible graphite sheet
material was maintained at 600.degree. C. in an electric furnace
for 4 hours. From the weight difference between before and after
the heat treatment, there was calculated an oxidation loss. Results
are shown in Table 1, together with data of tensile strength and
specific resistance.
EXAMPLE 3
100 g of North Korea-produced natural flake graphite particles
having a bulk density of 0.64 and a sieve size of 42 to 80 mesh
(Tyler) were dispersed in 400 g of 80% nitric acid, and then, 8 g
of potassium permanganate were portion-wise added, while stirring,
at 30.degree. C. As the potassium permanganate was added as
mentioned above, the temperature of the system rose by about
10.degree. C. After the addition of magnesium permanganate, the
system was heated, and maintained at 60.degree. C. for 3 hours to
accomplish oxidation reaction. The liquid in the system was removed
by centrifugation, and subsequently the resulting graphite
particles were washed with water so that the pH value thereof
became 6.0. The thus washed graphite particles were dehydrated by
centrifugation so that the water content thereof became about 30%,
to obtain wet graphite particles.
100 g of the wet graphite particles were dipped in 500 ml of an
aqueous solution (concentration: 3% by weight) of magnesium salt of
the boric acid ester of sucrose for 1 hour. The resulting soggy
graphite particles were subjected to suction filtration so that the
content of the aqueous solution was adjusted to 30% by weight.
The graphite particles treated as mentioned above were dried at
temperatures below 100.degree. C. and then heated in an electric
furnace at 1,000.degree. C. for 1 minute to obtain expanded,
vermiform graphite masses having a bulk density of 0.009.
5 g of the vermiform graphite masses were charged into a 100
mm.times.150 mm metal mould and then compression-molded under a
pressure of 100 Kg/cm.sup.2 by means of a pressing machine. The
compressed graphite masses were heated at 800.degree. C. and
further compressed under a pressure of 100 Kg/cm.sup.2, followed by
pressing by means of a constant speed roll so that the surface of
the product was flattened and smoothened. There was obtained a
flexible graphite sheet material having a thickness of 0.3 mm. A
sample piece (30 mm.times.60 mm) of the flexible graphite sheet
material was maintained at 600.degree. C. in an electric furnace
for 4 hours. From the weight difference between before and after
the heat treatment, there was calculated an oxidation loss. Results
are shown in Table 1, together with data of tensile strength and
specific resistance.
EXAMPLE 4
100 g of Madagascar-produced natural flake graphite particles
having a bulk density of 0.81 and a sieve size of 50 to 80 mesh
(Tyler) were dispersed in 400 g of 75% nitric acid, and then, 9 g
of sodium permanganate trihydrate were portion-wise added, while
stirring, at 30.degree. C. As the sodium permanganate trihydrate
was added as mentioned above, the temperature of the system rose by
about 10.degree. C. After the addition of magnesium permanganate,
the system was heated, and maintained at 60.degree. C. for 2 hours
to accomplish oxidation reaction. The liquid in the system was
removed by centrifugation, and subsequently the resulting graphite
particles were washed with water so that the pH value thereof
became 6.0. The thus washed graphite particles were dehydrated by
centrifugation so that the water content thereof became about 30%,
to obtain wet graphite particles.
100 g of the wet graphite particles were dipped in 500 ml of an
aqueous solution (concentration: 3% by weight) of magnesium salt of
the boric acid ester of sucrose for 1 hour. The resulting soggy
graphite particles were subjected to suction filtration so that the
content of the aqueous solution was adjusted to 30% by weight.
The graphite particles treated as mentioned above were dried at
temperatures below 100.degree. C. and then heated in an electric
furnace at 1,000.degree. C. for 1 minute to obtain expanded,
vermiform graphite masses having a bulk density of 0.011.
5 g of the vermiform graphite masses were charged into a 100
mm.times.150 mm metal mould and then compression-molded under a
pressure of 100 Kg/cm.sup.2 by means of a pressing machine. The
compressed graphite masses were heated at 800.degree. C. and
further compressed under a pressure of 100 Kg/cm.sup.2, followed by
pressing by means of a constant speed roll so that the surface of
the product was flattened and smoothened. There was obtained a
flexible graphite sheet material having a thickness of 0.3 mm. A
sample piece (30 mm.times.60 mm) of the flexible graphite sheet
material was maintained at 600.degree. C. in an electric furnace
for 4 hours. From the weight difference between before and after
the heat treatment, there was calculated an oxidation loss. Results
are shown in Table 1, together with data of tensile strength and
specific resistance.
Table 1 ______________________________________ Tensile Specific
Example Specific Oxidation strength resistance, No. gravity loss, %
Kg/cm.sup.2 .mu..OMEGA.cm ______________________________________ 1
1.2 37.6 65 600 2 1.2 31.5 66 600 3 1.2 30.2 69 550 4 1.2 34.0 69
600 Comparative Ex. 1 1.2 52.2 62 550 Comparative Ex. 2 1.2 80.7 60
500 ______________________________________
EXAMPLES 5 TO 10
100 g of Madagascar-produced natural flake graphite particles
having a bulk density of 0.81 and a sieve size of 50 to 80 mesh
(Tyler) were dispersed in 400 g of 75% nitric acid, and then, 7 g
of potassium permanganate were portion-wise added, while stirring,
at 30.degree. C. As the potassium permanganate was added as
mentioned above, the temperature of the system rose to about
40.degree. C. After the addition of magnesium permanganate, the
system was heated, and maintained at 60.degree. C. for 2 hours to
accomplish oxidation reaction. The liquid in the system was removed
by centrifugation, and subsequently the resulting graphite
particles were washed with water so that the pH value thereof
became 6.0. The thus washed graphite particles were dehydrated by
centrifugation so that the water content thereof became about 30%,
to obtain wet graphite particles.
On the other hand, using each of the metal salts of the boric acid
esters obtained in Referential Examples 1 to 2 and 4 to 7, there
were prepared six kinds of aqueous solutions each having a solid
concentration of 3% by weight.
Using each of the thus prepared aqueous solutions, antioxidizing
treatment was carried out as follows.
100 g of the wet graphite particles as prepared above were dipped
in 500 ml of an aqueous solution of the metal salt of the boric
acid ester for 1 hour. The resulting soggy graphite particles were
subjected to suction filtration so that the content of the aqueous
solution was adjusted to 30% by weight.
The graphite particles treated as mentioned above were dried at
temperatures below 100.degree. C. and then heated in an electric
furnace at 1,000.degree. C. for 1 minute to obtain expanded,
vermiform graphite masses having a bulk density of 0.012.
5 g of the vermiform graphite masses were charged into a 100
mm.times.150 mm metal mould and then compression-molded under a
pressure of 100 Kg/cm.sup.2 by means of a pressing machine. The
compressed graphite masses were heated at 800.degree. C. and
further compressed under a pressure of 100 Kg/cm.sup.2, followed by
pressing by means of a constant speed roll so that the surface of
the product was flattened and smoothened. There was obtained a
flexible graphite sheet material having a thickness of 0.3 mm. Two
sample pieces (30 mm.times.60 mm) of the flexible graphite sheet
material was maintained, in an electric furnace for 4 hours, at
400.degree. C. and at 600.degree. C., respectively. From the weight
difference between before and after the heat treatment, there was
calculated an oxidation loss. Results are shown in Table 2,
together with data of tensile strength and specific resistance.
Table 2
__________________________________________________________________________
No. of the Referential Example in which the employed metal Example
salt of boric acid ester was Specific Oxidation loss, % Tensile
strength, Specific resistance No. prepared gravity 400.degree.
C./4hr 600.degree. C./4hr Kg/cm.sup.2 .mu..OMEGA.cm
__________________________________________________________________________
5 Referential Example 1 1.2 2.8 36.8 68 550 6 Referential Example 2
1.2 3.0 37.0 66 580 7 Referential Example 4 1.2 2.4 36.0 67 620 8
Referential Example 5 1.2 2.3 35.8 66 600 9 Referential Example 6
1.2 2.8 36.5 67 620 10 Referential Example 7 1.2 3.0 36.8 65 550
__________________________________________________________________________
* * * * *